1. Field of the Invention
The present invention relates to a traction block for a linear winch of the type comprising an elongate frame having a U shaped cross section having a flat web and two lateral spaced apart flanges perpendicular to the web, two clamping blocks of elongate shape, which are mounted for movement between the lateral flanges of the frame so that a relative longitudinal movement between each movable clamping block and the adjacent flange of the frame causes a transverse movement of the respective movable clamping block, and a flat cover plate parallel to the web of the frame and fixed removably thereto.
2. Description of the Prior Art
Linear winches are well known apparatus for exerting a pull on a cable, a bar or other elongated traction means, for example for moving a heavy load horizontally or vertically. A linear winch usually comprises two traction blocks which alternately clamp the cable, a first one of the two traction blocks being usued for pulling the cable while said cable slides between the clamping blocks of the second traction block, and the latter being used for holding the cable while the first traction block is brought back to its starting position.
In the prior traction blocks manufactured by the applicant (see for example European patent No. 0 057 622) and one embodiment of which is shown in FIG. 1 of the accompanying drawings, the web 2 of frame 1 is formed by a metal plate and the flanges 3 of the frame are formed by bearing blocks. Each bearing block 3 comprises on its inner face a roller track 4 on which one of the two movable clamping blocks 5 may roll through a plurality of rollers 6. Each roller track 4 forms an angle with the longitudinal axis of the traction block, i.e. with the longitudinal axis of the cable, of the bar or other traction means 7 on which the traction block is intended to act, the roller tracks 4 of the two bearing blocks 3 converging towards one of the two ends of the traction block and each movable clamping block 5 having the shape of a wedge having an apex angle which is equal to the angle formed between the adjacent roller track 4 and the longitudinal axis of the traction block. The cover plate 8 is formed by a plate similar to plate 2 of frame 1. Plate 2, the two bearing blocks 3 and the cover plate 8 are held assembled by two series of stud bolts 9, the stud bolts 9 of one of the two series passing through holes provided in one of the two bearing blocks 3, whereas the stud bolts 9 of the other series pass through holes provided in the other bearing block 3 (a single stud bolt of each series is shown in FIG. 1).
In use, when the movable clamping blocks 5 clamp cable 7, each of the two bearing blocks 3 is subjected, by reaction, to a very considerable force in the direction of arrow F.sub.1 or in the direction of arrow F.sub.2 depending on the bearing block 3 considered. By way of example, if the pitch of each roller track 4 is 10%, each bearing block 3 is subjected to a force which is equal to about five times the traction force of the linear winch, i.e. for example 300 tons for a traction force of 60 tons. The addition to their assembling role, stud bolts 9 must also be capable of withstanding such a high transverse force. That is why, in the embodiment shown in FIG. 1, a relatively large number of assembling stud bolts 9 must be associated with each bearing block 3, usually 6 to 10 stud bolts depending on the power of the linear winch, i.e. a total of 12 to 20 stud for each traction block.
Now, in use it is desirable for the cover plate 8 of each of the two traction blocks of the linear winch to be able to be easily and rapidly removed for withdrawing or moving the movable clamping block sufficiently aside for setting the traction blocks of the linear winch on the cable 7 at any point thereof without having to thread a long length of the cable between the movable clamping blocks 5. That is also the case when cable 7 is formed from two or more cable sections joined end to end by joining means and when it is desired to pass a joining means through one of the traction blocks while the cable is held under tension by the other traction block.
It is clear that, in the case of the embodiment shown in FIG. 1, for removing cover plate 8 all the stud bolts 9, usually 12 to 20 stud bolts, must be unscrewed which complicates removal of cover plate 8.
In FIG. 2 of the accompanying drawings another embodiment of a prior art traction block is shown. In FIG. 2, the elements which are identical or which play the same role as in FIG. 1 are designated by the same reference numbers. In the embodiment shown in FIG. 2, the cover plate 8 is in the form of a massive piece which may slide in the longitudinal direction with respect to the bearing blocks 3 of frame 1. One or more pins 11 prevent cover plate 8 from sliding with respect to the bearing blocks 3 during use. In use, the pin or pins 11 are not subjected to any transverse force. At the outside, a single pin would be sufficient for locking cover plate 8 with respect to the bearing blocks 3. In the embodiment shown in FIG. 2, cover plate 8 may be easily and rapidly removed since it is sufficient to remove one or a small number of pins 11 and to slide cover 8 longitudinally with respect to the bearing blocks 3. However, in this embodiment, the transverse forces (in the direction of arrows F.sub.1 and F.sub.2) which, in use, are applied by reaction to the bearing blocks 3, are absorbed by the lateral flange 8a of cover plate 8. The result is that the junction zones 8b between the lateral flanges 8a and part 8c of cover plate 8 are subjected to very high moments of flexure. Consequently, cover plate 8 must be a thick and solid piece. By way of comparison, for a linear winch having a traction force of 60 tons, cover plate 8 of the traction block of FIG. 2 has a thickness of 90 mm and a weight of about 280 kg, whereas cover plate 8 of the traction block of FIG. 1 has a thickness of about 40 mm and a weight of about 95 kg.